1. Field of the Invention
This invention relates to electrochemical devices and, more specifically, to devices known as electrolytic tilt sensors.
2. Description of the Related Art
Electrolytic tilt sensors include devices that provide an output voltage proportional to tilt angle and a phase indicative of tilt direction when it is configured as part of an appropriate electrical circuit. The output voltage derives from the resistance within the electrolyte (also referred to as the "electrolytic solution") of the tilt sensor, which is a function of the tilt of the electrolyte due to the gravitational or other force. Tilt sensors, originally conceived for weapon delivery and aircraft navigation, have found a range of uses, including the monitoring of drill head angles in remote locations (such as wells) and the leveling of construction laser systems used in architectural alignment. This is primarily because the tilt sensor's voltage signal output may provide an input related to tilt angle to a pre-programmed guidance or other system, or provide an indicia of tilt angle via an electrical signal at a location away from the sensor.
An electrolytic tilt sensor is typically comprised of a glass cell that is partially filled with an electrolyte and three or more electrodes (including one common electrode) extending through the cell so that they are at least partially immersed in the electrolyte. The portion of the cell not filled with the electrolyte is a gaseous bubble, which shifts as the cell is tilted, also causing the electrolyte to shift. Consequently, the electrodes become more or less immersed with the electrolyte as the bubble shifts. This shift provides a change in impedance between any one electrode and the common electrode. When the tilt sensor electrodes are configured as part of an appropriate electrical circuit, the angle of tilt may be correlated to an output voltage of the circuit.
As noted, the fundamental output of an electrolytic tilt sensor configured within an electrical circuit is an output voltage that is correlated to the tilt angle. Thus, in order for the tilt sensor to work accurately and reliably over time, all the electrical parameters of the components of the circuit, such as the applied voltage and the resistors within the circuit, must be stable over time. Most importantly, the resistivity of the electrolyte must remain stable in order for the output voltage to remain accurately correlated to tilt angle. Thus, it is generally desired that the resistivity of the electrolyte in a commercially tilt sensor not vary by more than 25% over 1000 hours of usage for temperatures between -20.degree. C. and +50.degree. C. and that the tilt sensor output have a sensitivity of 7 millivolts per arc-minute and a response time of 1 second or less for a change in tilt angle. In addition, electrical null voltage should coincide with mechanical null voltage by no more than 3.degree.. Null voltage is the minimum output as measured under standard test conditions with the tilt sensor at or near the horizontal level position. The mechanical null voltage is the voltage output when the tilt sensor is positioned with respect to a stable horizontally oriented reference surface. The change in the resistivity (or equivalent) of the electrolyte over time is referred to as the "load life".
There are, however, many electrochemical reactions, described in further detail below, that arise when a voltage is applied across an electrolyte (via metal electrodes) that give rise to a change in resistivity and, when used in conjunction with a tilt sensor, a deviation in output voltage for a given tilt angle. Where this change occurs over a relatively short period of operational time, the tilt sensor will be, at best, of little use and, at worse, dangerous (for example, when used in an aircraft navigational system). Precious metal electrodes have been used almost exclusively due to their chemical stability, which served to suppress some of these reactions sufficiently, and slow the change in resistivity of the electrolyte. However, because of their expense, electrolytic tilt sensors using precious metal electrodes did not find widespread commercial usage.
The use of non-precious metal electrodes in tilt sensors has not been successful. Although there may have been suggestions of a variety of tilt sensors using non-precious metal electrodes, nothing in the prior art divulged how to overcome the rapid electrolytic system breakdown (i.e., change in resistivity) in a tilt sensor when a non-precious metal electrode is used. While certain electrolytes which are inherently slow to degrade electrochemically may likewise have been suggested in the prior art, nothing in the prior art suggests their use with non-precious metal electrodes. Nor have additional operating parameters been previously identified that would result in a commercially viable non-precious metal electrode tilt sensor. Consequently, there have been no commercially successful tilt sensors that use non-precious metal electrodes.
For example, U.S. Pat. No. 4,028,260 to Zuest ("Zuest") refers to the use of non-precious metal electrodes (including thin metal precious metal electrodes), and discusses some of the electrochemical reactions that degrade performance. Nothing in Zuest, however, suggests that the devices would be reliable after 100 hours of operation, far short of the 1000 hour minimum generally required for a commercial tilt sensor. Furthermore, the Zuest electrolytes require the toxic material hydrazine monohydrate. Without this material, which would be have to be omitted for widespread commercial acceptance of a tilt sensor, the Zuest electrolytes degrade within a few hours. Thus, Zuest presents a very specifically composed electrolyte for use-with non-precious metal electrodes, which would not give rise to a commercially viable tilt sensor.
Similarly, U.S. Pat. No. 3,843,539 to Willing and Cooper ("Willing et al.") describes an electrolyte that uses ammonium carbonate in ethanol, methanol and a variety of higher molecular weight alcohols. Willing et al., however, focuses on a different facet of the tilt sensor art, specifically, properties that facilitate construction and calibration of the devices. In use, the device of Willing et al. as described would fail because of electrochemical reactions that would result in rapid degradation of the electrolyte and electrodes. Willing et al. fails to address or even recognize the difficulties of creating an operational tilt sensor. Furthermore, Willing et al. does not mention the use of non-precious metal electrodes, which tends to accelerate the degrading electrochemical reactions and complicate the task of constructing a viable commercial tilt sensor. Nor does Willing et al. raise the possibility of impressing specific operational parameters in order to minimize the electrochemical degradation.
It would be extremely beneficial to provide a low-cost electrolytic tilt sensor constructed with non-precious metal electrodes and an electrolytic solution which does not degrade during use.